Shearography

A vacuum shearography hood and data display unit is applied to a composite structure to check for defects. An indication of a possible defect appears as the ripple pattern on the bottom right of the data screen.

Shearography or speckle pattern shearing interferometry is a measuring and testing method similar to holographic interferometry. It uses coherent light or coherent soundwaves^[1] to provide information about the quality of different materials in nondestructive testing, strain measurement, and vibration analysis. Shearography is extensively used in production and development in aerospace,^[2] wind rotor blades, automotive, and materials research areas.^[3] Advantages of shearography are the large area testing capabilities (up to 1 m² per minute), non-contact properties, its relative insensitivity to environmental disturbances, and its good performance on honeycomb materials, which is a big challenge for traditional nondestructive testing methods.

Shearing function

When a surface area is illuminated with a highly coherent laser light, a stochastical interference pattern is created. This interference pattern is called a speckle, and is projected on a rigid camera's CCD chip. Analogous with Electronic speckle pattern interferometry (ESPI), to obtain results from the speckle we need to compare it with a known reference light. Shearography uses the test object itself as the known reference; it shears the image so a double image is created. The superposition of the two images, a shear image, represents the surface of the test object at this unloaded state. This makes the method much less sensitive to external vibrations and noise. By applying a small load, the material will deform. A nonuniform material quality will generate a nonuniform movement of the surface of the test object. A new shearing image is recorded at the loaded state and is compared with the sheared image before load. If a flaw is present, it will be seen.^[4]

Phase-shift technology

To increase the sensitivity of the measurement method, a real-time phase shift process is used in the sensor. This contains a stepping mirror that shifts the reference beam, which is then processed with a best fit-algorithm and presents the information in real time.

Applications

The main applications are in composite nondestructive testing, where typical flaws are:

• Disbonds
• Delaminations
• Wrinkles
• Porosity
• Foreign objects
• Impact damages

Industries where Shearography is used are:

• Aerospace
• Space
• Boats
• Wind power
• Automotive
• Tires
• Art conservation ^[5]

Inspection standards

The methododlogy of shearography is standardized by ASTM International:

• ASTM E2581-07, "Standard Practice for Shearography on Polymer Matrix Composites, Sandwich Core Materials and Filament Wound Pressure Vessel’s in Aerospace Applications"

The following NDT personnel certification documents contain references to shearography:

• BS EN 4179:2009
• NAS 410, 2008 Rev 3
• ASNT SNT-TC-1A, 2006 edition
• ASNT CP-105, 2006 edition

References

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